Welcome, fair traveller to the Deep Dark, where no sunlight shines upon us and danger lurks in every corner!
If you happen to git clone this repo, this may help you work with the codebase.
Elytra Trims uses Stonecutter to manage multiple Minecraft versions. Familiarize yourself with it if you want to work on the code.
TL;DR: use task stonecutter/Set active version to X before running Minecraft on X.
You can use project/buildActive to build that version specifically.
Do not run build task, project/chiseledBuild is the correct one.
Do not run reformat on random files or worse the whole project. It will remove important versioned comments and break the build.